Notarizing DMG with app bundle nested in folder

I'm building an app that is nested inside a folder in the distributed DMG. To be clear, this means that when the user opens the DMG, they will see the DMG contents which will contain a folder, and within that folder they will find my app bundle.

When I run the notarization command, I get this output:

$ xcrun altool --validate-app \
  --file dist/example-project.dmg \
  --primary-bundle-id com.example.MyBundleIdentifier \
  --username $AC_USERNAME \
  --password $AC_PASSWORD
2020-02-26 16:24:41.176 altool[5275:117673] *** Error: Unable to validate archive 'dist/example-project.dmg': (
    "Error Domain=ITunesSoftwareServiceErrorDomain Code=-21017 \"Could not find the main bundle or the Info.plist is missing a CFBundleIdentifier in 'dist/example-project.dmg'.\" UserInfo={NSLocalizedDescription=Could not find the main bundle or the Info.plist is missing a CFBundleIdentifier in 'dist/example-project.dmg'., NSLocalizedFailureReason=Unable to validate your application.}"
)

I can open the DMG and verify that it has the app bundle nested within a folder. Googling variations of "notarize dmg" has not helped, and `xcrun altool --help` does not reveal any flags that I can use to specify the bundle path within the DMG.

How do I notarize a DMG with an app bundle nested within a folder?

Thanks in advance.